This invention relates to wound treatment using an outer wound covering which both thermally insulates the wound and surrounding skin area to minimize heat loss and conducts heat from surrounding skin to the wound in order to maintain the wound close to normal body temperature.
There are two types of common chronic wounds. One type termed a bed sore or pressure sore is caused by constant pressure applied to a part of a body when a patient has limited mobility in a hospital bed, wheel chair, etc. The pressure point reduces circulation to that part of the body resulting in lowering of temperatures (hypothermia), reduced physiological activity, and finally the breakdown of tissue at the pressure point causing a wound. A second type of chronic wound is caused by reduced vascular activity at the wound site which reduces the flow of oxygenated blood to the wound area (arterial) and or accumulation of body fluid at the wound site (venus). This second type of chronic wound has an underlying cause such as diabetes or artherosclerosis. Due to the lack of adequate blood circulation to the wound area, this area is typically at a lower temperature relative to healthy skin surrounding the wound. Healthy skin exhibits surface temperatures which are several degrees lower than the body core temperature of 37xc2x0 C. due to heat loss to the environment. The temperature of the surface of a wound can be significantly lower than that of normal skin, sometimes approaching temperatures as low as the surrounding room air. The application of heat to a wound has been shown to increase blood flow to the wound thereby increasing oxygen concentration at the wound site, giving increased rates of fluid and waste removal from the wound and increased immune function.
The therapeutic application of heat to a wound has been common practice since ancient times. Various types of artificial heating have been used on wounds including such devices as hot water bottles, warming pads, and heating lamps. These kinds of therapies can result in the accidental burning of the wound or surrounding skin area or the excessive drying out of the wound. There has also been concern amongst medical practitioners that heating a wound above the core body temperature can result in accelerated growth of bacteria due to both the elevated temperature and the reduced capacity of a chronic wound to fight infection.
Heat loss from a wound is the result of heat transfer from the wound to the surrounding environment. Heat transfer can occur in three basic ways. The first type, conductive heat transfer, occurs when energy is transported from one body to another due to molecular vibration and interaction. An example of this kind of heat transfer is the hand touching a cold piece of metal where the molecules of the skin of the hand lose energy by inducing vibration in the atoms of the metal. The second kind of heat transfer is convection which is heat energy transport due to bulk fluid motion along the surface of a solid. An example is the xe2x80x9cwind chill factorxe2x80x9d where the motion of the air hitting the body removes additional heat and makes the outdoor temperature feel colder than it really is. Finally there is radiation heat transfer which results from the emission of electromagnetic waves from a surface. Heat eminating from a light bulb is an example of this kind of heat transfer. The third type of heat transfer is usually only significant at temperatures above 200 C. and is a minor component-of overall heat transfer at room temperature up to normal body temperature. The body, which maintains a constant core temperature, utilizes blood flow as a heat transfer medium similar to water in a car""s radiator. As it flows, the blood heats the body""s extremities such as the leg, foot, or arm. At the same time the skin of these extremities is constantly losing heat to the environment primarily due to conduction and convection heat transfer. Chronic wounds, having impaired blood flow, tend to lose heat to the environment without getting enough heat replenishment from the blood.
Minimizing heat loss from a wound is an alternative method to maintain an elevated wound temperature without resorting to external heating devices. Westby et al (U.S. Pat. No. 5,531,670, 1996) describes a heat conserving bandage consisting of one or more heat reflecting layers which reflect radiation heat eminating from the body back in the direction of the body. The bandage wraps around muscle and bone tissue and is used for heat therapy on muscles or other internal organs on both animals and humans. It is not intended for use on skin wounds, which due to their location on the exterior of the body tend to lose significantly more heat than internal organs such as muscle. All the examples of therapy using the Westby patent related to internal organs such as muscle, ligaments etc. According to the patentees, the primary mechanism for heat preservation is the reflection of radiation heat back to the source. As stated above, radiation heat transfer is small compared to the other two heat transfer mechanisms for temperatures below 200xc2x0 C. The patentees also mention the use of closed cell rubber as a thermal conduction insulator. There are much better barriers to conduction heat transfer available than the rubber material. Using the Westly bandage for skin wounds may result in substantial heat loss due to the location of the skin at the exterior of the body.
Adequate heat transfer at the site of a wound is the subject of several patents. Eidus (U.S. Pat. No. 3,596,657, August 1971) describes a thermally conductive surgical dressing consisting of conventional cotton gauze interwoven with high conductive metallic thread. The metallic thread offers high thermal conductivity for removing heat from a surgical wound or injecting heat into a surgical wound using either ice packs or heating packs. Augustine et al (U.S. Pat. No. 6,235,047, May 2001) describes a thermally conductive bandage consisting of a polymeric layer which absorbs water enhancing thermal conductivity. Both devices basically sit on the wound and immediate skin area (periwound) only. They do not extend to other skin areas in the general area of the wound. Since the Eidus device utilizes a mesh of cotton interwoven with metal thread, the thermal conductivity will still be significantly lower than for mesh made entirely of metal or a metal sheet. The Augustine device uses water as the primary thermal conductor. Water is a good thermal conductor compared to air but a very poor thermal conductor compared to all high thermal conductive metals such as silver, copper, gold, and aluminum. Aluminum which is the least efficient conductor of the metals mentioned above, still has a thermal conductivity which is almost 400 times greater than pure water (237 W/M oK for water). Neither of these two patents is intended to conserve body heat and distribute it to the wound. Both use external heat sources to heat the wound.
Medical research has found that the best conditions for wound healing are those which duplicate as closely as possible normal physiological conditions. This includes wound temperature, which is optimized when it is as near to the normal core physiological temperature of the human body (37xc2x0 C.) as possible.
Due to the inadequate transfer of oxygenated blood to the area of chronic wounds, the temperature of the wound is normally below that of healthy skin in nearby locations. Even healthy skin exhibits surface temperatures which are below the body core temperature due to heat loss. This problem is exacerbated for chronic wounds at locations where the soft tissue is normally very thin, as in the shin of the leg, big toe or heel of the foot, and other areas where bond protrusions are very close to the outer skin. In order to bring the wound temperature as close to the body core temperature (37xc2x0 C.) without the use of externally applied artificial heat, the present invention utilizes basic principles of heat transfer to both conserve body heat in the vicinity of the wound and to distribute it to the wound itself As mentioned earlier, the primary mechanism for heat loss in a wound and surrounding skin areas is conduction heat transfer. Radiation heat transfer only becomes significant at more elevated temperatures (200xc2x0 C.). Unlike inventions cited above, which deal primarily with the reflection of radiant energy back to the wound, the present invention deals with conduction heat transfer, utilizes high efficiency insulators to prevent body heat from escaping and high efficiency conductors for distributing stored heat energy to the wound area. The bandage in the present invention covers not only the wound area but major portions of skin and muscle mass surrounding the wound in order to maximize the amount of heat which is conserved and then redirect the heat to the wound area.
Properties of thermal insulators which minimize conductive heat loss are well known. The best insulators are those with the lowest thermal conductivity. A vacuum is the best thermal insulator. Next to a vacuum, gases are a close second and are much better thermal insulators than liquids or solids. Of readily available gases, several exhibit very low thermal conductivities while at the same time being safe to use around humans. These include air, nitrogen, argon, carbon dioxide, krypton, and possibly the Freon family (CC13F). These gases can be obtained in various types of configurations where the solid content and conductivity of the gas housing is minimized in order to minimize heat conduction due to the solid. Furthermore, geometric configurations of the insulators can be modified in order to minimize heat loss from the wound and the surrounding skin areas based on heat transfer theory. Thermal conductivity values for these low conductivity gases are given below.
Recent advances in materials science have resulted in a totally new class of materials called aerogels which have the lowest thermal conductivities of any solids available today. Aerogels are highly porous materials with pore sizes down in the nanometer range. Thermal conduction through the solid part of the aerogel is minimized by the small connections between particles making up the conduction path. Conduction by gas trapped in the pores of the aerogel is minimized because the extremely small pore size which is about the same length as the mean free path of the gas. Aerogels can be made from a variety of precurser materials including silica, alumina, titania, hafnium carbide, and carbon. Silica aerogel exhibit a typical thermal conductivity of about 0.017 W/M oK at 27xc2x0 C. and 1 atmosphere pressure. When the pressure is reduced to about 0.1 atmosphere the conductivity drops dramatically to 0.009 W/M oK. Silica aerogels have recently become available which are very flexible and can be twisted and wrapped around solid objects similar to cloth. Aerogel thermal conductivity can be further decreased by substituting a low conductivity gas such as carbon dioxide instead of air, in the pores of the aerogel. The present invention describes the use of the flexible aerogel to minimize heat loss near the wound.
Since healthy skin located near a chronic wound, especially skin in areas where significant amounts of muscle are present such as the calf, exhibits significantly higher temperatures than the wound itself (due to the impaired blood transport to the wound area), the transfer of heat from the healthy skin areas to the wound would help ameliorate the hypothermia of the wound area and bring it closer to normal core body temperature thereby promoting natural physiological healing. This can be accomplished by using a flexible conductive sheath which totally surrounds the wound area, adjacent healthy skin, and adjacent muscle mass. Conductivity in the sheath can be optimized by using metals which exhibit the highest values of thermal conductivity including silver, gold, copper, aluminum or alloys of these metals. The conductance of heat to the wound area can further be optimized by using a conductor geometry and thickness which maximizes conduction heat transfer while at the same time being plyable enough to be applied directly to the skin. This high efficiency thermal conductor sits directly on portion of healthy skin near the wound and directly on top of a wound bandage or dressing in the wound area. Conduction from the wound through the wound bandage to the metal sheath can be optimized by reducing wound bandage thickness and/or using bandage materials such as hydrogels which load up with water or adding colloidial silver to the bandage material to increase overall bandage conductivity. Typical conductances for the high efficiency conductors mentioned above are as follows:
A fairly new class of materials called carbon-carbon composites exhibit thermal conductivities 3 times as high as copper (1200 W/M oK). Using the carbon-carbon composites in formulating the conductive sheath can offer optimal heat transfer from the surrounding skin to the wound.
The combination of ultra high insulators to minimize heat loss from the wound area with high thermal conductors to maximize heat transfer to the wound area from surrounding healthy skin has been shown to increase wound temperature by as much as 3.7xc2x0 C. and bring the wound temperature close to the normal physiological core temperature. The following temperature data were obtained for the shin area of a healthy leg when using a prototype of the insulator-conductor wound covering of the present invention. The wound covering was placed over a gauze pad covering the skin (wound area) and totally wrapped around the calf muscle.
Ambient temperature: 74.7xc2x0 F. (23.7xc2x0 C.)
Temperature at surface of gauze pad: 92.0xc2x0 F. (33.3xc2x0 C.)
Average temperature of gauze pad covered with insulator-conductor wound covering over 6 hour period: 96.5xc2x0 F. (35.8xc2x0 C.)
Tests conducted with a prototype insulator-conductor boot which fits over the foot and extends up to the calf of the foot, has shown an average temperature at the big toe (after an equilibration period of 40 minutes) of 97.3xc2x0 F. (36.3xc2x0 C.) as compared to an average temperature of the big toe with gauze pad of 90.7xc2x0 F. (32.6xc2x0 C.).
The present invention gives various geometries for the insulator-conductor wound covering for use on various parts of the body, including leg, knee, foot, arm, elbow, hand, hip area, and buttocks area. Various configurations of the insulator are presented including thickness, type of insulator gas, type of gas containment device, and use of aerogel insulator materials. Various configurations of the conductor include thickness, type of metal used, and geometric configuration used. The configuration of the insulator pad as compared to the conductor pad is also presented as is the use with various kinds of common wound bandages.
Some advantages of the insulator-conductor wound treatment device over current wound heating methodologies are as follows:
1. Maintains wound temperature close to normal body temperature without the use of external heaters thereby promoting wound healing under the most ideal physiological conditions.
2. Wound covering fits directly over various types of wound bandages and is easy to apply and wear, giving the patient full freedom of mobility and allowing for normal daily routine.
3. Wound covering can be worn for many hours a day giving maximum therapeutic effect.
4. Wound covering can be attached to all common chronic wounds in various parts of the body including leg, knee, foot, arm, elbow, hand, buttocks, and hip area.
5. Wound covering totally eliminates the possibility of electrical shock from electrical appliance attached to the patient.
6. Wound covering eliminates the possibility of overheating resulting in burning of skin tissue.
7. Wound covering eliminates the need for frequent visits to the clinic for application of heat therapy.
8. Wound covering is an easy to use as an elastic bandage thereby promoting acceptance by medical practitioners.
9. Wound covering stores and distributes heat generated from increased metabolism and blood flow due to activities such as walking, cleaning and bicycling thereby providing extra heat to the wound over an extended period of time.
10. Wound covering promotes natural healing close to the ideal physiological temperature.